KiDS+VIKING-450 and DES-Y1 combined: Cosmology with cosmic shear

TL;DR

This study performs a joint cosmic shear analysis of KV450 and DES-Y1, addressing redshift-distribution systematics by homogenizing priors and nonlinear modeling and by spectroscopically calibrating DES-Y1 redshifts via the DIR method. Using MICE2 mock catalogs to assess potential DIR biases, the authors demonstrate that redshift calibration shifts can notably affect $S_8$, with the DES-Y1 recalibration lowering its inferred $S_8$ and the combined KV450 + DES-Y1 constraint reaching $S_8 = 0.762^{+0.025}_{-0.024}$, in tension with Planck 2018 at $2.5\sigma$. The analysis achieves a precision improvement by a factor of about $\sqrt{2}$ over the individual surveys and underscores redshift calibration as a crucial systematic for current and future weak-lensing surveys, while outlining paths to further tighten constraints via broader scale coverage, additional surveys like HSC-Y1, and more realistic mock-based systematics.

Abstract

We present a combined tomographic weak gravitational lensing analysis of the Kilo Degree Survey (KV450) and the Dark Energy Survey (DES-Y1). We homogenize the analysis of these two public cosmic shear datasets by adopting consistent priors and modeling of nonlinear scales, and determine new redshift distributions for DES-Y1 based on deep public spectroscopic surveys. Adopting these revised redshifts results in a $0.8σ$ reduction in the DES-inferred value for $S_8$, which decreases to a $0.5σ$ reduction when including a systematic redshift calibration error model from mock DES data based on the MICE2 simulation. The combined KV450 + DES-Y1 constraint on $S_8 = 0.762^{+0.025}_{-0.024}$ is in tension with the Planck 2018 constraint from the cosmic microwave background at the level of $2.5σ$. This result highlights the importance of developing methods to provide accurate redshift calibration for current and future weak lensing surveys.

Figures (4)

• Figure 1: Marginalized posterior contours in the $S_8$ -- $\Omega_{\mathrm m}$ plane (inner 68% CL, outer 95% CL). We show the KV450 constraints in green (solid) using an analysis setup that follows Hildebrandt19, but including an additional redshift dependence of the IA signal (denoted 'KV450'). In black (dashed), we show the DES-Y1 constraints corresponding to the original Troxel18 analysis, noting that the sum of neutrino masses is varied in this analysis (and hence the contour should not be directly compared with the orange (solid) Planck 2018 contour where neutrino mass is fixed). The blue (solid) contours show the DES-Y1 constraints where an identical setup to the KV450 analysis is used (along with the original DES-Y1 redshift distributions).
• Figure 2: DES-Y1 redshift distributions for the four tomographic bins (in black, blue, cyan, red, respectively), showing the publicly released distributions (dashed) and the spectroscopically determined distributions using the DIR approach (solid). The distributions based on spectroscopy are systematically shifted to larger redshifts compared to the original distributions (accounting for $\Delta z_i$), and hence favor a lower value of $S_8$ compared to the original DES-Y1 analysis in Troxel18. See Table \ref{['tab1label']} for the mean redshifts of the different tomographic bins for the two approaches. The vertical dotted lines denote the tomographic bin boundaries. The small inset shows the redshift distribution of the matched photometry/spectroscopy catalogue for DES-Y1 containing approximately $30,000$ objects used in the DIR method. The spectroscopic calibration samples are obtained from zCOSMOS, VVDS-Deep (2h), CDFS, DEEP2 (2h), VVDS-Wide (22h). We do not show the uncertainties in the $n(z)$ for visual clarity (instead see Table \ref{['tab1label']} for uncertainties in the mean redshifts).
• Figure 3: Marginalized posterior contours in the $S_8$ -- $\Omega_{\mathrm m}$ plane (inner 68% CL, outer 95% CL) for KV450 (green), DES-Y1 following a spectroscopic calibration of the redshift distributions and identical setup to the KV450 analysis (purple), the above combined (pink), and Planck 2018 (orange).
• Figure 4: Marginalized posterior contours in the $S_8$ -- $\Omega_{\mathrm m}$ plane (inner 68% CL, outer 95% CL) following an alternative analysis of the cosmic shear datasets with MICE2 priors on the $\Delta z_i$ parameters. We show KV450 in green, DES-Y1 in purple, KV450 + DES-Y1 in pink, and Planck 2018 in orange.